Chronic Gq signaling in AgRP neurons does not cause obesity

Reciprocal activity of AgRP and POMC neurons governs coordinated control of feeding and metabolism

Agouti-related peptide (AgRP)-expressing and proopiomelanocortin (POMC)-expressing neurons reciprocally regulate food intake. Here, we combine non-interacting recombinases to simultaneously express functionally opposing chemogenetic receptors in AgRP and POMC neurons for comparing metabolic responses in male and female mice with simultaneous activation of AgRP and inhibition of POMC neurons with isolated activation of AgRP neurons or isolated inhibition of POMC neurons. We show that food intake is regulated by the additive effect of AgRP neuron activation and POMC neuron inhibition, while systemic insulin sensitivity and gluconeogenesis are differentially modulated by isolated-versus-simultaneous regulation of AgRP and POMC neurons. We identify a neurocircuit engaging Npy1R-expressing neurons in the paraventricular nucleus of the hypothalamus, where activated AgRP neurons and inhibited POMC neurons cooperate to promote food consumption and activate Th+ neurons in the nucleus tractus solitarii. Collectively, these results unveil how food intake is precisely regulated by the simultaneous bidirectional interplay between AgRP and POMC neurocircuits.

Neurosecretory Protein GM-Expressing Neurons Participate in Lipid Storage and Inflammation in Newly Developed Cre Driver Male Mice

Obesity induces inflammation in the hypothalamus and adipose tissue, resulting in metabolic disorders. A novel hypothalamic neuropeptide, neurosecretory protein GM (NPGM), was previously identified in the hypothalamus of vertebrates. While NPGM plays an important role in lipid metabolism in chicks, its metabolic regulatory effects in mammals remain unclear. In this study, a novel Cre driver line, NPGM-Cre, was generated for cell-specific manipulation. Cre-dependent overexpression of Npgm led to fat accumulation without increased food consumption in male NPGM-Cre mice. Chemogenetic activation of NPGM neurons in the hypothalamus acutely promoted feeding behavior and chronically resulted in a transient increase in body mass gain. Furthermore, the ablated NPGM neurons exhibited a tendency to be glucose intolerant, with infiltration of proinflammatory macrophages into the adipose tissue. These results suggest that NPGM neurons may regulate lipid storage and inflammatory responses, thereby maintaining glucose homeostasis.

Advanced neurobiological tools to interrogate metabolism

Engineered neurobiological tools for the manipulation of cellular activity, such as chemogenetics and optogenetics, have become a cornerstone of modern neuroscience research. These tools are invaluable for the interrogation of the central control of metabolism as they provide a direct means to establish a causal relationship between brain activity and biological processes at the cellular, tissue and organismal levels. The utility of these methods has grown substantially due to advances in cellular-targeting strategies, alongside improvements in the resolution and potency of such tools. Furthermore, the potential to recapitulate endogenous cellular signalling has been enriched by insights into the molecular signatures and activity dynamics of discrete brain cell types. However, each modulatory tool has a specific set of advantages and limitations; therefore, tool selection and suitability are of paramount importance to optimally interrogate the cellular and circuit-based underpinnings of metabolic outcomes within the organism. Here, we describe the key principles and uses of engineered neurobiological tools. We also highlight inspiring applications and outline critical considerations to be made when using these tools within the field of metabolism research. We contend that the appropriate application of these biotechnological advances will enable the delineation of the central circuitry regulating systemic metabolism with unprecedented potential.

Obesity-associated microglial inflammatory activation paradoxically improves glucose tolerance

Hypothalamic gliosis associated with high-fat diet (HFD) feeding increases susceptibility to hyperphagia and weight gain. However, the body-weight-independent contribution of microglia to glucose regulation has not been determined. Here, we show that reducing microglial nuclear factor κB (NF-κB) signaling via cell-specific IKKβ deletion exacerbates HFD-induced glucose intolerance despite reducing body weight and adiposity. Conversely, two genetic approaches to increase microglial pro-inflammatory signaling (deletion of an NF-κB pathway inhibitor and chemogenetic activation through a modified Gq-coupled muscarinic receptor) improved glucose tolerance independently of diet in both lean and obese rodents. Microglial regulation of glucose homeostasis involves a tumor necrosis factor alpha (TNF-α)-dependent mechanism that increases activation of pro-opiomelanocortin (POMC) and other hypothalamic glucose-sensing neurons, ultimately leading to a marked amplification of first-phase insulin secretion via a parasympathetic pathway. Overall, these data indicate that microglia regulate glucose homeostasis in a body-weight-independent manner, an unexpected mechanism that limits the deterioration of glucose tolerance associated with obesity.

Hunger potentiated

Dieting often fails in the long run becuase of an ever-growing urge to eat. In this issue of Cell Metabolism, Grzelka et al. unveil a brain circuit that is potentiated during caloric restriction and incites rebound increases in food consumption and body weight.

Nutrient-sensing AgRP neurons relay control of liver autophagy during energy deprivation

Autophagy represents a key regulator of aging and metabolism in sensing energy deprivation. We find that fasting in mice activates autophagy in the liver paralleled by activation of hypothalamic AgRP neurons. Optogenetic and chemogenetic activation of AgRP neurons induces autophagy, alters phosphorylation of autophagy regulators, and promotes ketogenesis. AgRP neuron-dependent induction of liver autophagy relies on NPY release in the paraventricular nucleus of the hypothalamus (PVH) via presynaptic inhibition of NPY1R-expressing neurons to activate PVH^CRH neurons. Conversely, inhibiting AgRP neurons during energy deprivation abrogates induction of hepatic autophagy and rewiring of metabolism. AgRP neuron activation increases circulating corticosterone concentrations, and reduction of hepatic glucocorticoid receptor expression attenuates AgRP neuron-dependent activation of hepatic autophagy. Collectively, our study reveals a fundamental regulatory principle of liver autophagy in control of metabolic adaptation during nutrient deprivation.

A nociceptive neuronal ensemble in the dorsomedial prefrontal cortex underlies pain chronicity

Pain chronicity involves unpleasant experience in both somatosensory and affective aspects, accompanied with the prefrontal cortex (PFC) neuroplastic alterations. However, whether specific PFC neuronal ensembles underlie pain chronicity remains elusive. Here we identify a nociceptive neuronal ensemble in the dorsomedial prefrontal cortex (dmPFC), which shows prominent reactivity to nociceptive stimuli. We observed that this ensemble shows distinct molecular characteristics and is densely connected to pain-related regions including basolateral amygdala (BLA) and lateral parabrachial nuclei (LPB). Prolonged chemogenetic activation of this nociceptive neuronal ensemble, but not a randomly transfected subset of dmPFC neurons, induces chronic pain-like behaviors in normal mice. By contrast, silencing the nociceptive dmPFC neurons relieves both pain hypersensitivity and anxiety in mice with chronic inflammatory pain. These results suggest the presence of specific dmPFC neuronal ensembles in processing nociceptive information and regulating pain chronicity.

Food-induced dopamine signaling in AgRP neurons promotes feeding

Obesity comorbidities such as diabetes and cardiovascular disease are pressing public health concerns. Overconsumption of calories leads to weight gain; however, neural mechanisms underlying excessive food consumption are poorly understood. Here, we demonstrate that dopamine receptor D1 (Drd1) expressed in the agouti-related peptide/neuropeptide Y (AgRP/NPY) neurons of the arcuate hypothalamus is required for appropriate responses to a high-fat diet (HFD). Stimulation of Drd1 and AgRP/NPY co-expressing arcuate neurons is sufficient to induce voracious feeding. Delivery of a HFD after food deprivation acutely induces dopamine (DA) release in the ARC, whereas animals that lack Drd1 expression in ARC^AgRP/NPY neurons (Drd1^AgRP-KO) exhibit attenuated foraging and refeeding of HFD. These results define a role for the DA input to the ARC that encodes acute responses to food and position Drd1 signaling in the ARC^AgRP/NPY neurons as an integrator of the hedonic and homeostatic neuronal feeding circuits.

Dopamine-inhibited POMCDrd2+ neurons in the ARC acutely regulate feeding and body temperature

Dopamine acts on neurons in the arcuate nucleus (ARC) of the hypothalamus, which controls homeostatic feeding responses. Here we demonstrate a differential enrichment of dopamine receptor 1 (Drd1) expression in food intake-promoting agouti related peptide (AgRP)/neuropeptide Y (NPY) neurons and a large proportion of Drd2-expressing anorexigenic proopiomelanocortin (POMC) neurons. Owing to the nature of these receptors, this translates into a predominant activation of AgRP/NPY neurons upon dopamine stimulation and a larger proportion of dopamine-inhibited POMC neurons. Employing intersectional targeting of Drd2-expressing POMC neurons, we reveal that dopamine-mediated POMC neuron inhibition is Drd2 dependent and that POMCDrd2+ neurons exhibit differential expression of neuropeptide signaling mediators compared with the global POMC neuron population, which manifests in enhanced somatostatin responsiveness of POMCDrd2+ neurons. Selective chemogenetic activation of POMCDrd2+ neurons uncovered their ability to acutely suppress feeding and to preserve body temperature in fasted mice. Collectively, the present study provides the molecular and functional characterization of POMCDrd2+ neurons and aids our understanding of dopamine-dependent control of homeostatic energy-regulatory neurocircuits.

Regulation of body weight and food intake by AGRP neurons during opioid dependence and abstinence in mice

Dysregulation of body weight maintenance and opioid dependence are often treated as independent disorders. Here, we assessed the effects of both acute and long-term administration of morphine with and without chemogenetic activation of agouti-related peptide (AGRP)-expressing neurons in the arcuate nucleus (ARCAGRP neurons) to elucidate whether morphine and neuronal activation affect feeding behavior and body weight. First, we characterized interactions of opioids and energy deficit in wild-type mice. We observed that opioid administration attenuated both fasting-induced refeeding and ghrelin-stimulated feeding. Moreover, antagonism of opioid receptors blocked fasting-induced refeeding behavior. Next, we interfaced chemogenetics with opioid dependence. For chemogenetic experiments of ARCAGRP neurons, we conducted a priori behavioral qualification and post-mortem FOS immunostaining verification of arcuate activation following ARCAGRP chemogenetic activation. We administered clozapine during short-term and long-term morphine administration paradigms to determine the effects of dependence on food intake and body weight. We found that morphine occluded feeding behavior characteristic of chemogenetic activation of ARCAGRP neurons. Notably, activation of ARCAGRP neurons attenuated opioid-induced weight loss but did not evoke weight gain during opioid dependence. Consistent with these findings, we observed that morphine administration did not block fasting-induced activation of the ARC. Together, these results highlight the strength of opioidergic effects on body weight maintenance and demonstrate the utility of ARCAGRP neuron manipulations as a lever to influence energy balance throughout the development of opioid dependence.

Development of "Hunger Neurons" and the Unanticipated Relationship Between Energy Metabolism and Mother-Infant Interactions

Over the course of a lifetime, the perinatal period plays an outsized role in the function of physiological systems. Here, we discuss how neurons that regulate energy metabolism contribute to the infant's relationship with the mother. We focus our discussion on Agrp neurons, which are located in the arcuate nucleus of the hypothalamus. These neurons heavily regulate energy metabolism. Because offspring transition from a period of dependence on the caregiver to independence, we discuss the importance of the caregiver-offspring relationship for the function of Agrp neurons. We present evidence that in the adult, Agrp neurons motivate the animal to eat, while in the neonate, they motivate the offspring to seek the proximity of the caregiver. We specifically highlight the peculiarities in the development of Agrp neurons and how they relate to the regulation of metabolism and behavior over the course of a lifetime. In sum, this review considers the unique insights that ontogenetic studies can offer toward our understanding of complex biological systems, such as the regulation of energy metabolism and mother-infant attachment.

The DREADDful Hurdles and Opportunities of the Chronic Chemogenetic Toolbox

The chronic character of chemogenetics has been put forward as one of the assets of the technique, particularly in comparison to optogenetics. Yet, the vast majority of chemogenetic studies have focused on acute applications, while repeated, long-term neuromodulation has only been booming in the past few years. Unfortunately, together with the rising number of studies, various hurdles have also been uncovered, especially in relation to its chronic application. It becomes increasingly clear that chronic neuromodulation warrants caution and that the effects of acute neuromodulation cannot be extrapolated towards chronic experiments. Deciphering the underlying cellular and molecular causes of these discrepancies could truly unlock the chronic chemogenetic toolbox and possibly even pave the way for chemogenetics towards clinical application. Indeed, we are only scratching the surface of what is possible with chemogenetic research. For example, most investigations are concentrated on behavioral read-outs, whereas dissecting the underlying molecular signature after (chronic) neuromodulation could reveal novel insights in terms of basic neuroscience and deregulated neural circuits. In this review, we highlight the hurdles associated with the use of chemogenetic experiments, as well as the unexplored research questions for which chemogenetics offers the ideal research platform, with a particular focus on its long-term application.

Molecular Tools for Targeted Control of Nerve Cell Electrical Activity. Part II

In modern life sciences, the issue of a specific, exogenously directed manipulation of a cell's biochemistry is a highly topical one. In the case of electrically excitable cells, the aim of the manipulation is to control the cells' electrical activity, with the result being either excitation with subsequent generation of an action potential or inhibition and suppression of the excitatory currents. The techniques of electrical activity stimulation are of particular significance in tackling the most challenging basic problem: figuring out how the nervous system of higher multicellular organisms functions. At this juncture, when neuroscience is gradually abandoning the reductionist approach in favor of the direct investigation of complex neuronal systems, minimally invasive methods for brain tissue stimulation are becoming the basic element in the toolbox of those involved in the field. In this review, we describe three approaches that are based on the delivery of exogenous, genetically encoded molecules sensitive to external stimuli into the nervous tissue. These approaches include optogenetics (overviewed in Part I), as well as chemogenetics and thermogenetics (described here, in Part II), which is significantly different not only in the nature of the stimuli and structure of the appropriate effector proteins, but also in the details of experimental applications. The latter circumstance is an indication that these are rather complementary than competing techniques.

Rolling out physical exercise and energy homeostasis: Focus on hypothalamic circuitries

Energy balance is the fine regulation of energy expenditure and energy intake. Negative energy balance causes body weight loss, while positive energy balance promotes weight gain. Modern societies offer a maladapted way of life, where easy access to palatable foods and the lack of opportunities to perform physical activity are considered the roots of the obesity pandemic. Physical exercise increases energy expenditure and, consequently, is supposed to promote weight loss. Paradoxically, physical exercise acutely drives anorexigenic-like effects, but the mechanisms are still poorly understood. Using an evolutionary background, this review aims to highlight the potential involvement of the melanocortin system and other hypothalamic neural circuitries regulating energy balance during and after physical exercise. The physiological significance of these changes will be explored, and possible signalling agents will be addressed. The knowledge discussed here might be important for clarifying obesity aetiology as well as new therapeutic approaches for body weight loss.

AgRP neurons: Regulators of feeding, energy expenditure, and behavior

Neurons in the hypothalamic arcuate nucleus (ARC) that express agouti-related peptide (AgRP) govern a critical aspect of survival: the drive to eat. Equally important to survival is the timing at which food is consumed-seeking or eating food to alleviate hunger in the face of a more pressing threat, like the risk of predation, is clearly maladaptive. To ensure optimal prioritization of behaviors within a given environment, therefore, AgRP neurons must integrate signals of internal need states with contextual environmental cues. In this state-of-the-art review, we highlight recent advances that extend our understanding of AgRP neurons, including the neural circuits they engage to regulate feeding, energy expenditure, and behavior. We also discuss key findings that illustrate how both classical feedback and anticipatory feedforward signals regulate this neuronal population and how the integration of these signals may be disrupted in states of energy excess. Finally, we examine both technical and conceptual challenges facing the field moving forward.

Chronic glucocorticoid treatment induces hepatic lipid accumulation and hyperinsulinaemia in part through actions on AgRP neurons

Glucocorticoids (GCs) are widely prescribed anti-inflammatory medicines, but their use can lead to metabolic side-effects. These may occur through direct actions of GCs on peripheral organs, but could also be mediated by the hypothalamic AgRP neurons, which can increase food intake and modify peripheral metabolism. Therefore, the aim of this study was to examine the metabolic effects of chronic treatment with the GC corticosterone (Cort, 75 μg/ml in drinking water) in mice lacking the glucocorticoid receptor (GR) on AgRP neurons. Female AgRP-GR KO mice had delayed onset of Cort-induced hyperphagia. However, AgRP-GR KO had little impact on the increased body weight or adiposity seen with 3 weeks Cort treatment. Cort caused hepatic steatosis in control mice, but in Cort treated female AgRP-GR KO mice there was a 25% reduction in liver lipid content and lower plasma triglycerides. Additionally, Cort treatment led to hyperinsulinaemia, but compared to controls, Cort-treated AgRP-GR KO mice had both lower fasting insulin levels and lower insulin levels during a glucose tolerance test. In conclusion, these data indicate that GCs do act through AgRP neurons to contribute, at least in part, to the adverse metabolic consequences of chronic GC treatment.

Chemogenetic approaches to identify metabolically important GPCR signaling pathways: Therapeutic implications

DREADDs (Designer Receptors Exclusively Activated by a Designer Drug) are designer G protein-coupled receptors (GPCRs) that are widely used in the neuroscience field to modulate neuronal activity. In this review, we will focus on DREADD studies carried out with genetically engineered mice aimed at elucidating signaling pathways important for maintaining proper glucose and energy homeostasis. The availability of muscarinic receptor-based DREADDs endowed with selectivity for one of the four major classes of heterotrimeric G proteins (G_s , G_i , G_q , and G₁₂ ) has been instrumental in dissecting the physiological and pathophysiological roles of distinct G protein signaling pathways in metabolically important cell types. The novel insights gained from this work should inform the development of novel classes of drugs useful for the treatment of several metabolic disorders including type 2 diabetes and obesity.

Gut-to-brain signals in feeding control

Interoceptive signals from gut and adipose tissue and sensory cues from the environment are integrated by hubs in the brain to regulate feeding behavior and maintain homeostatic control of body weight. In vivo neural recordings have revealed that these signals control the activity of multiple layers of hunger neurons and eating is not only the result of feedback correction to a set point, but can also be under the influence of anticipatory regulations. A series of recent technical developments have revealed how peripheral and sensory signals, in particular, from the gut are conveyed to the brain to integrate neural circuits. Here, we describe the mechanisms involved in gastrointestinal stimulation by nutrients and how these signals act on the hindbrain to generate motivated behaviors. We also consider the organization of multidirectional intra- and extrahypothalamic circuits and how this has created a framework for understanding neural control of feeding.